1. Field of the Invention
The invention relates to a method for monitoring the conversion rate of an exhaust catalyst intended for an internal combustion engine.
2. Description of the Prior Art
DE German reference 44 33 988 A1 discloses a catalyst monitoring procedure which records exhaust-gas temperatures upstream and downstream of the catalyst. The rate of heat transfer from the exhaust gas to the catalyst material is continually calculated, and from this the catalyst temperature and the rate of change of this catalyst temperature are determined. The quotient from the rate of heat transfer and the rate of change of the temperature of the catalyst is determined for a specific period of time, this period of time being chosen from the cold start of the internal combustion engine to the reaching of the conversion temperature of the catalyst. On the basis of the variation over time of the quotient, a trend is continually determined for the time up until reaching the conversion temperature. The time elapsed up until reaching the conversion temperature is compared with a threshold value for this period of time and, if the threshold value is exceeded, an error signal is output.
One of the procedures disclosed in German reference DE 42 11 092 A1 is based on a temperature estimate for the catalyst on the basis of a temperature model. In the case of that procedure, it is assumed for the model that there is a still just usable catalyst after reaching the temperature at which conversion begins and for the heat produced in the conversion. For simulating the temperature behavior of the catalyst, in that procedure it is initially assumed that the heat capacity of the catalyst is known and, furthermore, the amount of heat supplied to the engine is estimated. This takes place by recording the amount of fuel fed to the engine and calculating the proportion of heat energy which is fed to the catalyst. In this case, the proportion of heat energy depends on the respective load state and the speed of the engine. Consequently, the amount of heat which is fed to the catalyst is recorded for comparatively small units of time from the amount of fuel, the load and the speed. These amounts of heat are summated, in order in this way to obtain a total amount of heat which the catalyst receives from the start of the procedure. In addition, the amount of heat dissipated from the catalyst to the surroundings is estimated. The temperature estimated with this amount of heat model is subsequently compared with its actual temperature. This procedure, which is also intended to be applicable in the driving mode of a vehicle, is only started if the catalyst is at approximately ambient temperature at the beginning of the procedure. From the start of the procedure, a catalyst temperature is estimated with the aid of a temperature model block, using signals of the load, speed, injection times, air temperature, air pressure and octane number of the fuel. This temperature is compared with a stored comparison temperature. It is subsequently further investigated whether the conversion temperature of the catalyst has already been reached.